Swifts and Swallows
Bird feathers are very delicate and the natural oils on humans hands can strip them of their weatherproofing. Avoid handling and wear gloves.
Nestlings leave the nest fully flighted and do not spend time on the ground like other songbirds. All 4 toes face forward to cling to vertical surfaces. It is difficult for them to take flight from the ground. If worried about a swift or swallow place it feet first against a tree and watch to make sure they are able to take flight.